1. Field of the Invention
The present invention relates to the electrization technology to the coil in the focus control.
2. Description of the Related Art
Conventionally, it is known to employ a stepping motor for a driving source of an autofocus (“AF”) controller used for an optical apparatus, such as a camera. In addition, it is also known that the AF uses a phase-difference detection method, an external ranging method, a contrast detection method, or a hybrid method (Japanese Patent Laid-Open No. 2004-109690) that combines the foregoing methods.
The phase-difference detection method splits a luminous flux from a subject, introduces the two split luminous fluxes into two AF sensors, and calculates a defocus amount based on a shift amount or interval between two images output from these sensors. The external ranging method is classified into an active method and a passive method. The active method irradiates an infrared ray or an ultrasonic wave to a subject, detects the reflected wave through a sensor, and measures a distance to the subject based on a time period necessary for the reflected wave to return and an irradiation angle. On the other hand, the passive method splits a luminous flux from a subject, receives the split luminous fluxes with two light receiving sensors (line sensors), and operates a correlation between signals from two sensors. The passive method operates the number of pixels of the line sensor which corresponds to a shift amount that maximizes the correlation, and obtains focusing information based on the shift amount on the triangulation principle. The contrast detection method extracts a high frequency component in a video signal output from an imaging section by reciprocating or wobbling a focus lens, and moves the focus lens to a position that provides a maximum value to obtain the on-focus state.
Other prior art include Japanese Patent Laid-Open No. 09-331666.
The stepping motor is likely to step out due to high-speed driving or load fluctuations. In addition, although the phase-difference detection method or the external ranging method drives the focus lens at a high speed to a target position after obtaining the defocus amount or focusing information, and shortens a focusing time period, the stepping out can occur when these methods use the stepping motor. If a motor having a large step width or a low reduction gear ratio of a transmission mechanism is used for high-speed driving with the stepping motor, a high resolution is hard to obtain and the focusing accuracy deteriorates. In addition, it restricts a wobbling width in the contrast detection system, and degrades the quality of a recorded image. In addition, a high driving resolution of a focus lens is effective so as to improve the focusing accuracy and to provide the degree of freedom to the wobbling width in the contrast detection method. On the other hand, when a motor with a small step width or an enhanced reduction gear ratio of a transmission mechanism is used to obtain a high resolution with a stepping motor, high-speed driving is difficult and a focusing time period is likely to increase.